This invention relates to solid waste and more particularly, relates to equipment and method for treating solid waste for the purpose of producing commercially valuable by-products such as separated plastics and pulverized refuse derived fuel from solid waste.
Solid waste includes municipal wastes, industrial wastes, agricultural wastes and mixtures of these solid wastes. For the purpose of illustration, however, the invention is described mostly in connection with presorted municipal solid waste.
Treating or disposing of solid waste is an extremely serious and ever worsening national and international problem. This is because: (1) solid waste is rapidly growing in volume and tonnage even on a per capita basis; (2) the rapidly rising cost of landfill and other ecological considerations; and (3) the skyrocketing cost of energy and other valuable materials which the solid waste contains. Table 1 clearly shows all these three trends for municipal solid waste.
Table 1 ______________________________________ Trends in Municipal Solid Waste Year 1920 1940 1960 1977 2000 ______________________________________ Waste production rate: 2.6 3.1 4.3 5.4 7.5 per capita, lbs/day Total potential recovery: Energy equivalent, million bbls of oil 130 160 210 270 375 Energy, billion dollars -- -- -- 4.1 -- (1977 $15/bbl) Steel, million tons 8 9.5 12 17 22 Steel, million dollars (1977 $50/ton) -- -- -- 850 -- Plastic/rubber, million tons 5.5 7 8 11 16 Plastic/rubber, billion dollars -- -- -- 2.2 -- (1977 $200/ton) ______________________________________
The very huge amount of solid wastes must be especially treated in an environmentally acceptable way, at any cost. When such treatment is properly done, not only can the harmful solid waste be converted into less harmful or neutral materials for easy disposal, but ideally the different components of the solid waste can be separated, and individually recovered for sale and recycling.
Table 1 shows the economic value of municipal solid waste. The resources of municipal solid waste are indeed very rich. In the average municipal solid waste, the following recoverable and resaleable components are present:
1. Metals, 8% by weight
2. Glass, 12% by weight
3. Plastics, 6% by weight
4. Fibrous combustible matter, 60% by weight
Fibrous combustible matters are abundant in municipal solid waste. They are in the form of waste paper, newspaper, magazines, carton boxes, or other miscellaneous paper in thicknesses from 0.001" to 1/4" or more. The average heat content (or heat of combustion) of the fibrous combustible matters from municipal solid waste is about 5,000 Btu/lb. If completely recovered in the form of refuse derived fuel, the heat content gives the equivalent of 270 millions of barrels of oil in the year 1977 alone. At the oil price of $15 per barrel, this is equivalent to $4.1 billion.
Another source of fibrous combustible matters is the biomass, which is a combination of solid waste, agricultural waste, and plant life including trees, shrubs, grass cuttings, weeds or stalks of wheat, rice, and sugar cane. The total heat content in the biomass if all recovered could be as much as 30 times that in municipal solid waste, i.e., the equivalent of 8.1 billion barrels of oil, or over $120 billion in 1977 alone.
The cost of landfill now ranges from $3 to $18 per ton, but this cost is increasing by about 8% per year, the same rate as energy costs increase. In addition, the Resource Conservation Recovery Act of 1976 requires that all states in this country close all dumps by 1983; thus the cost of the remaining landfill space will probably rise still higher and at even faster rates. Furthermore, the energy proposals by President Carter and the Congress contemplate higher costs for both domestic oil and newly discovered natural gas. It is, therefore, very important to transform solid waste into valuable, storageable fuel and other materials. This gives rise to the new "resource recovery" industry.
It is estimated that with 10 years, resource recovery will blossom into a billion dollar industry. About 30%, perhaps up to 50%, of the 147 million tons of municipal solid waste generated each year will be converted into fuel to supply up to 5% of the electricity needs for utilities and 18% of the gas needs in certain geographic areas.
Various technologies for recovering the resources from municipal solid waste have been developed and tried, with varying degrees of success. One technology involves "bulk burning" the solid waste in incinerators to produce steam for electricity, heat or cooling. This technology produces a non-storageable fuel, requires relatively high capital costs for the expensive combustion equipment, but relatively low operating costs. Another technology as disclosed for example, in U.S. Pat. No. 3,961,913 involves first shredding the solid waste and then mechanically separating the heavy fraction comprising metals and glass from the light fraction consisting essentially of paper and plastics. The separated metals and glass may be sold or landfilled. The paper (or other fibrous combustible matter) is treated chemically to weaken or break the fibrous bonds and then pulverized in air-filled hot ball mills with 450.degree. F. steel balls to produce a refuse derived fuel. The plastic is mixed in the fuel and causes a serious pollution problem. This technology produces storageable "refuse derived fuel" and, compared to the first technology, has lower capital costs but higher operating costs because of the shredding operation. A third technology, called pyrolysis, involves burning the solid waste with very little oxygen to produce synthetic oil or gas, either for sale or for conversion into steam. This third technology has high capital and operating costs, partly because of the frequent jammings in the treating equipment, and is very experimental.
All these technologies suffer many disadvantages:
1. These technologies are unreliable and non-reproducible because each of them involves a number of processing steps which are not easily controlled or completely understood, but are very much dependent on the composition of the solid waste.
2. The treating equipments are bulky and heavy, costly to build, difficult to maintain, and irregular in operation.
3. The treating equipments are slow in operation. In the second technology, for example, the treating temperature is limited to about 450.degree. F., above which paper and other fibrous combustible matter readily burn in air. This low treatment temperature requires long treatment time, i.e., over 40 minutes, to debond the fibrous matter.
4. They are high energy-consuming processes. Again, in the second technology, for example, hot ball mills are usually used to disintegrate the chemically treated fibrous matter to make the refuse derived fuel. The hot balls have to be raised against gravity from the bottom of the mills to near the top of the mills before such raised balls will drop down to strike other balls with the treated fibrous matter in between. The energy used to raise the balls is not only wasted, but harmfully wasted to produce harsh sounds on impact at more than 95 dBA.
5. Fusion of the plastics with other plastics, with other materials in the solid waste, or with the wall or components of the treating equipment often causes serious jamming problems, briefly mentioned previously, or even causes destruction of the equipment.
6. The non-separated, or incompletely separated, plastics, particularly PVC plastics, produce on burning not only dense smoke, but cancerous or poisonous fumes. This smoke and the fumes together with the noise and other pollution problems, often force the waste-treating industry to abandon the projects because of local opposition. Even finding a site can be difficult.
7. In treating wastes to obtain refuse derived fuel, an oxidizing or combustion supporting atmosphere is used inside the hot 450.degree. F. treating equipment. This atmosphere, usually air, causes surface oxidation or combustion that produces a fire-retarding surface layer of ash. The unwanted oxidation and combustion further destroys the heat and economic values of the refuse derived fuel and presents, in addition, a distinct fire or explosion hazard. Because of the non-separation or incomplete separation of plastics from the original fibrous combustible matter from which the recovered fuel is derived, the derived fuel is not homogenous but varies between and even within lots as to average size, size distribution, density, composition, combustibility, jamming characteristics and, above all, the heat value in the fuel, e.g., Btu/lb., which makes the derived fuel difficult to sell and use. As a matter of fact, such derived fuel has never been used alone in electricity generation facilities.
As a result of the above and other difficulties, none of the present three technologies have bee profitable. All three still have unknown futures. Even their feasibility or operability and practical values have not been determined or estimated.